Alzheimer's disease (AD) is a neurodegenerative disorder characterized by memory loss, synaptic dysfunction and accumulation of amyloid β-peptides (Aβ). It is caused in part by increased levels of amyloid-β-peptide 1-42 (Aβ42). Phosphodiesterase 5 (PDE5) inhibitors are widely used drugs against erectile dysfunction and pulmonary hypertension. These inhibitors are believed to increase cGMP levels which enhances phosphorylation of the transcription factor and memory-affecting molecule cAMP-responsive element binding (CREB) through activation of cGMP-dependent-protein kinases.
Cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), nucleotide biological second messengers, regulate various biological processes, such as blood flow regulation, cardiac muscle contraction, cell differentiation, neural transmission, glandular secretion, and gene expression. Intracellular receptors for these molecules include cyclic nucleotide phosphodiesterases (PDEs), cyclic nucleotide dependent protein kinases (PG K), and cyclic nucleotide-gated channels. PDEs are a large family of proteins that catalyze the hydrolysis of 3′,5′-cyclic nucleotides to the corresponding 5′ monophosphates. There are eleven related, but biochemically distinct, human PDE gene groups. Some PDEs are specific for hydrolysis of cAMP (such as PDE4, PDE7, and PDE8), and some are cGMP specific (such as PDE5, PDE6, and PDE9), while some PDEs have mixed specificity (such as PDE1, PDE2, PDE3, PDE10, and PDE11).
PDE 5 inhibitors are cyclic guanosine 3′,5′-monophosphate type five cGMP PDE inhibitors, which include, but are not limited to, sildenafil, tadalafil, zaprinast, and vardenafil. PDE5 inhibitors increase cGMP levels by inhibiting the degradative action of PDE5 on cGMP. No PDE inhibitor has reached the marketplace for diseases of the CNS, and no PDE5 inhibitors have been used for the treatment of AD.